1. Field of the Invention
The present invention relates to a magnetizing motor, and more particularly, to a magnetizing motor capable of improving the magnetizing efficiency of a permanent magnet by discontinuously applying an alternating current to a magnetizing coil that magnetizes a permanent magnet for a constant time, and a magnetizing method thereof.
2. Description of the Background Art
Generally, a prior art magnetizing motor is provided with a permanent magnet body at a rotor having a bar conductor, and serves as an induction motor before an rpm of the rotor reaches a synchronous speed of a rotational magnetic field. The magnetizing motor serves as a permanent magnet motor by magnetizing the permanent magnet body so as to reach the synchronous speed of the rotational magnetic field.
FIG. 1 is a circuit diagram showing a magnetizing motor of the prior art.
As shown, a main induction coil 22a, a sub induction coil 23a, and a magnetizing coil 24a are respectively connected to a power supply 110 for supplying an alternating current in parallel. A first capacitor 131 for controlling a phase difference of a current applied to the sub induction coil 23a is connected to the sub induction coil 23a in accordance with a current applied to the main induction coil 22a. A second capacitor 141 for controlling a phase difference of a current applied to the magnetizing coil 24a is connected to the magnetizing coil 24a in accordance with a current applied to the main induction coil 22a. 
A PCT sensor for protecting a circuit from a short or fire by having an increased resistance value is connected to the second capacitor 141 in serial.
A switch 143 for switching the circuit is connected to the magnetizing coil 24a in serial.
The operation of the prior art magnetizing motor will be explained.
When a rotor of the magnetizing motor is to be rotated, a current having a high phase difference is applied to the sub induction coil 23a via the first capacitor 131. Accordingly, an induction current is generated at the bar conductor of the rotor by an electromagnetic magnetization. Then, the bar conductor is rotated in a magnetic field generated by the sub induction coil 23a according to Fleming's left hand rule.
Then, a current having a phase difference slower than the current applied to the sub induction coil 23a by 90° is applied to the main induction coil 22a. Accordingly, a magnetic field is generated at a stator, and thus the bar conductor of the rotor is continuously rotated.
However, since the rotor is a rigid magnetic body having a high magnetic permeability, a magnetic field generated by the main induction coil 22a and the sub induction coil 23a magnetizes the body of the rotor. Accordingly, the body of the rotor receives hysteresis torque H by a hysteresis effect, and is thus rotated.
As shown in FIG. 2, the body of the rotor and the bar conductor receive a hysteresis torque H and an induction torque I, respectively, thereby being rotated by an integration torque of the hysteresis torque H and the induction torque I.
When an rpm of the body of the rotor and the bar conductor becomes a synchronous speed (3600 rpm) of the rotational magnetic field, the induction torque I of the bar conductor becomes zero. Accordingly, the rotor is rotated with an rpm slower than the synchronous speed of the rotational magnetic field. That is, a slip phenomenon is generated.
When the body of the rotor and the bar conductor have an rpm corresponding to 75%-80% of the synchronous speed, a strong current is applied to the magnetizing coil 24a. Accordingly, a strong flux generated from the magnetizing coil 24a is transmitted to a permanent magnet body covering an outer circumferential surface of the rotor body through an end of a magnetizing pole, thereby magnetizing the permanent magnet body.
Since the magnetizing pole is formed of a rigid magnetic body having a high magnetic permeability and an end thereof is tapered, a strong flux is transmitted to the permanent magnet body of the rotor without a loss thus to magnetize the permanent magnet body.
The permanent magnet body becomes a permanent magnet, and is thus rotated in a rotational magnetic field generated at the stator.
Even when the rpm of the rotor is increased to be equal to the synchronous speed (3,600 rpm) of the rotation magnetic field, the permanent magnet is continuously rotated in the rotational magnetic field. Accordingly, a rotational force of the rotor is not decreased.
The prior art magnetizing motor is rotated by the induction torque I and the hysteresis torque H in a low speed step, and is rotated by a permanent magnet torque P in a high speed step (synchronous speed: 3,600 rpm), accordingly, as the magnetizing pole magnetizes the permanent magnet body.
However, the prior art magnetizing motor has the following problem. That is, as a large current is continuously applied to the magnetizing coil so as to magnetize the permanent magnet body, thermal loss is generated from the magnetizing coil. Accordingly, driving efficiency of the motor is lowered.